Telephone equipment service personnel, or craftspersons, typically employ a hand-held test set which, when coupled to terminals of a telephone circuit to be tested, may be used to generate prescribed test signals and to access a selected telephone unit by the operation of one or more keys, buttons or switches contained within the test set.
Typically, the keypad of the telephone test set is comprised of a matrix (in the form of rows and columns) of key contact links, the crosspoints of which are electrically coupled to one another by the operation of the keys or pushbuttons of the keypad, so as to provide signals to an associate telephone dialling signal generator (usually in the form of an integrated telephone dialler chip), which generates the appropriate dialling signal (pulse or DTMF signal). Because the test set may be exposed to a variety of environments, its physical housing is normally waterproofed and, in general, ruggedized, in order to minimize damage due to shock and to present the intrusion of moisture and foreign matter.
Now, although augmenting the normal dialling signal functionality of the test set keypad by the addition of modified control circuitry and attendant switches would serve to increase the capabilities of the test set, such an approach would require providing extra holes in the physical case in which the telephone test circuitry is housed (for mounting the additional switches) and, in turn, would require a suitable mechanism for effectively sealing the housing. Simply put, the more holes that are provided in the test set housing (to mount the additional switches), the greater likelihood there is for the intrusion of moisture or foreign matter.
One approach that might be used to take advantage of the already existing keypad contact link matrix would be the installation of a "second function" or "shift" key, which could serve to selectively port the keypad contact links to either the telephone dialling signal generator, or to additional test circuitry, as desired. However, because of the way in which telephone dialling signal generator circuitry typically detects key-contact closures, it is generally difficult to operate such dialler circuitry in tandem with other electronic circuits which share or are controlled by a common keypad.
More specifically, the typical commercially available telephone dialling signal generator normally operates in a standby mode, with all of the keypad contact link inputs that lie along a first keypad axis (e.g. all of the "column" contact links) asserted `high` and with all of its keypad contact link inputs that lie along a second axis (e.g. all of the "row" contacts) resistively tied `low` and, additionally, connected to sensing transistor circuitry. In this typical configuration, no significant current flows until one of the keypad contact link crosspoints is closed by the operation (depression) of its associated key (e.g. keypad button). When a key is operated, the second axis (e.g. "row") position of that key may be immediately determined, but the chip still must "test" the contact links of the keypad in order to locate the first axis (e.g. column) position of the depressed key.
One possible technique that may be employed by the dialler circuitry to test the location of the depressed key would be to reverse the roles of the first and second axis contact links, such that all of the second axis contact links are asserted `high`, while pull-down resistors and sensing transistors are connected to the first axis contact links, thereby allowing the first axis (e.g. "column") position of the depressed key switch to be located.
Regardless of the mechanism for carrying out this functionality, a basic problem exists in that all of the dialling signal generator keypad inputs are, in reality, sometimes acting as inputs and sometimes acting as outputs. As a consequence, any other electronic device that supplies output signals that may be coupled to such circuitry is likely to interfere with its operation, and any device that may receive inputs from a terminal to which the contact links are connected is likely to suffer interference caused by the operation of the dialling signal generator circuit.